Through the years, attempts have been made to produce footwear that is both comfortable and exhibits improved performance. Many attempts have proved unsatisfactory, in that they have failed to produce the desired effectiveness. A major emphasis of these attempts has been to increase the cushioning and performance of an athletic shoe by making modifications to the midsole (the material which generally lies above the outsole and below the insole). The numerous attempts to provide superior cushioning in athletic shoes have led to at least two broad categories of developments.
One category utilizes different materials and configurations of the midsole to improve cushioning, as well as to provide selective stability. For example, materials of different hardness may be used, or a variety of devices may be encapsulated in the midsole to increase cushioning and stability. Typically, such midsoles are constructed of cellular ethyl vinyl acetate (EVA), polyurethane (PU), or a combination of both. While EVA has the advantage of being light-weight, and PU has the advantage of increased memory capabilities and resilience, the cellular structure of both materials has a tendency to break down and therefore, diminish the useful lifespan of the midsole, and thus, the shoe.
A second category of developments in midsoles includes those structures which have encapsulated an insert within the midsole material itself. The insert, usually made of plastic material that is harder than the midsole material, does to a limited degree increase the lifespan of the shoe sole since, unlike the cellular material, it does not break down. However, as with the first category of developments, in the second category, the insert is still designed to be encapsulated within either EVA or PU. Therefore, this structure does not completely eliminate the tendency of the cellular material to break down. Thus, the lifespan of these midsoles is still seriously limited by the lifespan of the primary midsole material itself.